As a biometric technique, fingerprint based identification has been successfully used in numerous applications, such as access control for buildings, verification of personal identity for portable communication, computing and network interface devices, and forensics. An electronic fingerprint device generally includes a fingerprint-sensing device that acquires fingerprint from a finger surface, and a fingerprint recognition device, which is used to process the data obtained with the fingerprint-sensing device.
A variety of methods, including optical, capacitive, thermal, RF-imaging, and mechanical sensing, have been used for fingerprint sensors. Among these methods, capacitive fingerprint sensing becomes popular with the development of fingerprint IC (Integrated Circuit) sensors that integrates the sensing and signal processing circuits. Capacitive IC sensors measure the capacitance between an array of metal plates on a silicon surface and finger skin. Larger capacitance is obtained for the fingerprint ridges that are slightly closer than the valleys, and thus an image of the fingerprint can be generated by measuring the distribution of capacitance.
Normally, the sensing process for capacitive fingerprint sensors includes capacitance sensing and Analog to Digital (A/D) conversion. In capacitance sensing, an electrical level signal is generated for the capacitance of each sensing point, and then with an A/D converter the level signals are converted to digital signals which can be accessed by a CPU through a communication circuit. In this process, the capacitance sensing and the A/D conversion can be independent. The overall fingerprint sensing speed is limited by capacitance sensing speed, scanning frequency, communication bandwidth, and A/D conversion sampling rate. Normally it needs a few seconds to obtain a good fingerprint image.
In the present invention, a capacitive fingerprint sensing circuit is introduced. The sensing circuit converts the capacitance value of each sensing point directly into digital signals without using A/D devices. It is an object of the present invention to provide a simple, inexpensive, and fast capacitive fingerprint sensing means that is resistive to noises.
A second object of the present invention is to provide a fingerprint sensing device that is able to work with a variety of voltages, so that it can be better used in portable, battery powered or passive devices such as smart cards and RFIDs.